This invention relates generally to insulated high-voltage conductors and, more particularly, to secondary ignition leads for large industrial internal combustion engines. Secondary ignition leads are used to conduct a high-voltage pulse from an ignition coil or other high voltage source to spark plugs or similar devices installed in engine cylinders. The term "secondary" and the related term "primary" are commonly used to distinguish the electrical connections to an ignition coil or similar device used to transform a relatively low voltage on the primary side of the device to a much higher voltage needed on the secondary side to produce an ignition spark.
The spark produced by the high-voltage pulse ignites a mixture of fuel and air in the engine cylinder. Combustion of the fuel moves a piston in the cylinder and rotates an engine crankshaft to which the piston is connected. Traditionally, secondary ignition leads have been unshielded, allowing easy access to the lead insulation and easy access to current and voltage probes for deriving information about the signals transmitted to the engine spark plug. These signals could trigger a timing light to determine and adjust the timing of the engine for optimum operation, and to supply a signal to an oscilloscope, to display such information as pulse shape, spark plug voltage firing levels, spark durations, frequency, number of restrikes, and open circuit firings. More complex engine analyzers use the same information picked up from the secondary lead, to analyze engine performance in more detail. More recently, safety of operation has become a more important issue and the only secondary lead approved by various approval agencies has been a shielded lead. Shielding was seen as the only way to prevent a secondary lead from arcing to an open grounded structure if the lead insulation broke down in the presence of an explosive mixture of gases near the engine. Therefore, shielding has become required for safety reasons, and to obtain the necessary agency approvals. External shielding has the additional advantage of providing increased physical strength to the secondary lead. Unfortunately, shielding totally precludes the detection of current in the lead, so timing lights, oscilloscopes and more complex engine analyzers are rendered inoperative.
Secondary leads in large engines have been shielded by means of a stainless steel braid with a TEFLON liner sleeve, installed over a silicone-insulated conductive lead. The steel braid inhibits undesired electromagnetic radiation from the ignition lead to the atmosphere and protects the insulated lead from external damage, such as physical damage sometimes inflicted by personnel climbing onto the engine. Although this structure is quite rugged, it has a number of disadvantages, one of which is that a conventional timing light cannot be used on the shielded secondary lead to determine how the engine cylinder timing is performing. A timing light is a strobe device commonly used to monitor the adjustment of the timing of one ignition pulse with respect to the angular position of the engine crankshaft and the associated position of the piston in the cylinder. The timing light uses a pickup device, such as an inductive or capacitive pickup to detect a current or voltage pulse in the ignition lead, and this technique is rendered difficult or ineffective by the presence of a grounded metal braid around the lead.
Another disadvantage of the stainless steel braid is that the spark signal cannot be monitored by an oscilloscope pickup for relative spark voltage for breakdown, to measure spark or arc duration, restrikes, misfires, and other wave shape information that can be easily obtained through an unshielded ignition lead. Further disadvantages are the relatively high cost of the braid and the need for continual maintenance to keep the shielded secondary lead and its associated hardware ground connection in good condition.
An additional significant concern relating to secondary ignition leads is safety from inadvertent ignition of leaking explosive gas. Large industrial engines are commonly fueled by natural gas, so there is always the potential for accumulation of gas near the engine and for an explosion to be ignited by a spark from a malfunctioning ignition lead. The conventional braided-shield ignition lead achieves an acceptable level of safety by including a TEFLON inner liner sleeve between the braid and the insulation around the lead itself. Safety considerations are given a high priority by testing and standards organizations and any new secondary ignition lead must satisfy the requirements of these organizations. Although the conventional braided lead is approved for use, it has been criticized for its inability to accommodate a standard timing light and the elimination of spark signal information. There have also been industry concerns about the maintenance needed to keep the shielded lead in good condition, to make sure it passes or stays within the requirements for approval.
Accordingly, there is still room for improvement in secondary ignition lead structures. Ideally, it would be desirable to provide a shielded lead that could be used with conventional timing lights, to provide spark signal wave/shape information, and that meets or exceeds safety requirements. The present invention is directed to this end.